ci: Improve test suite error aggregation logic
ci: Improve test suite error aggregation logic

Before, `tester.runTestSuites` only returned the error of the first test
suite that failed, even if multiple test suites failed. This commit
makes it so the errors are aggregated and compiled into a single error.

NetSys/quilt,kelda/kelda,kelda/kelda,quilt/quilt,quilt/quilt,NetSys/quilt,NetSys/quilt,kelda/kelda,quilt/quilt,NetSys/quilt,kelda/kelda,quilt/quilt,NetSys/quilt